Knowledge of ceramic superconducting compositions is of recent origin. Superconductivity itself was discovered by the Dutch scientist Heike Onnes in 1911 while he was studying the electrical properties of mercury at very low temperatures. In more recent times, Ogg (1946) observed superconductivity in ammonia solutions and proposed that superconductivity arose in these quenched metal-ammonia solutions because of mobile electron pairs. About 1973, it was determined that niobium metal and its alloys exhibited superconductivity when cooled to liquid helium (.about.4 K) temperatures. Later results raised this temperature as high as 23.sup.K (-250.degree. C.). Until recently, it was believed that this temperature represented a barrier and that superconductivity above this point was not possible. This conviction was based on the theoretical work of Bardeen, Cooper and Schieffer (BCS theory--1946) which predicted such a limit. In the early 1970's, several theoretical proposals suggested that the critical temperature for superconductivity could indeed be increased. These included V. L. Ginzberg, Usp. Fiz. Nauk., 101 185 (1970), and D. Allender, J. Bray, & J. Bardeen, Phys. Rev. B8, 4433 (1973). However, the lack of any revelations of superconductivity above 23.sup.K augmented the belief that indeed this temperature could not be exceeded. In December 1986, Bednorz and Mller announced the discovery (G. Bednorz and A. Mller, Z. Phys., B64 189 (1986)) of a new ceramic superconducting compound based on lanthanum, barium, and copper oxides, whose critical temperature for superconductivity was close to 35 K. By the following month, the critical temperature, T.sub.c, for the onset of superconductivity was raised to nearly 80.degree. K. by Chu and coworkers (M. K. Wu, J. R. Ashburn, C. J. Tang, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang and C. W. Chu, Phys. Rev. Lett. 58 908 (1987)). This was achieved by changing the composition to yttrium barium copper oxide, approximated by the formula: EQU Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.6.3
This formula, determined experimentally, is not exactly stoichiometric. It is believed that the lack of specific stoichiometry contributes most to the onset of superconductivity. Nevertheless, the exact mechanisms connecting superconductivity with chemical composition and stoichiometry are not completely coherent, even though they are receiving intensive study at this time. The most recent superconducting ceramic compositions announced to date include:
Bismuth Strontium Calcium Copper Oxide: EQU Bi.sub.2 Sr.sub.3-x Ca.sub.x Cu.sub.2 O.sub.8+y EQU T.sub.c =114.sup.K.
Thallium Calcium (Barium) Copper Oxide:
______________________________________ Tl Ba.sub.2 Ca Cu.sub.2 O.sub.7 Tl Ba.sub.2 Ca.sub.2 Cu.sub.3 O.sub.9 Tl Ba.sub.2 Ca.sub.3 Cu.sub.4 O.sub.11 Tl Ba.sub.2 Ca.sub.4 Cu.sub.5 O.sub.13 T.sub.c = 120 K. ______________________________________
The main advantage to superconducting compositions with higher T.sub.c (critical temperature for change from semiconductor to superconductor) values is that they should perform better, i.e.--carry higher currents, when cooled to liquid nitrogen temperatures ( 78 K.).
Another recently announced superconducting ceramic is based on a copper-free composition, vis: EQU BaO--K2O--Bi.sub.2 O.sub.3
This compound becomes superconducting at about 30 K. While copper-oxide superconductors exhibit layered structures (see below) that carry current efficiently only along certain planes, this new material is a three-dimensional network of bismuth and oxygen with properties that are much less sensitive to crystallographic direction.
The mechanism of superconductivity in such oxide-based ceramic materials is not at all well understood. Ogg's original contribution suggested that superconductivity arose in quenched metal-ammonia solutions because of mobile electron pairs. The concept accepted at present is similar (the BCS theory), and suggests that if a mobile electron propagates through a lattice structure, it will normally interact with the bound electrons of the lattice because of differences in the electron quantum-spin number. However, if two such electrons form a pair which are bound through opposite spin-pairing (Cooper pairs), then no quantum interaction of the bound pairs can occur with the electrons of the lattice (which still have an electron moment). That the BCS theory has some validity is shown by the following consideration. The so-called 1:2:3 compound, composed of Y-Ba-Cu-O atoms, is prepared by the solid state reaction of the requisite oxides, vis: EQU 2Y.sub.2 O.sub.3 +4BaO+6CuO+5O.sub.2 =4YBa.sub.2 Cu.sub.3 O.sub.6.5.
It is now established (C. N. Rao et al, Nature, 327 185 (1987)) that high T.sub.c superconductivity in the Y-Ba-Cu-O system originates from a compound of stoichiometry: YBa.sub.2 Cu.sub.3 O.sub.7-x, where "x" is a value less than 1.0. This compound has the structure of the ideal perovskite, YBa.sub.2 Cu.sub.3 O.sub.9. Thus, the superconductor--YBa.sub.2 Cu.sub.3 O.sub.7 has about 25% fewer oxygen atoms present in the lattice as compared to the idealized cubic perovskite structure. This massive oxygen deficiency means that instead of the conventional three-dimensional crystalline cubic-stacking array of the perovskite, a unique layered structure results. A loss of even more oxygen atoms in this structure gives rise to the semiconductor, YBa.sub.2 Cu.sub.3 O.sub.6. The chain of copper atoms associated with a chain of oxygen atoms is believed to be the key to superconducting behavior. The apparent oxidation state of the copper atoms is above 2.sup.+ but below 3.sup.+. Yet the above description is an idealized one. The actual distinct charge and structural conformation of the copper-oxygen layers has not yet been specifically delineated. Note that there appear to be extra oxygen atoms in the superconducting unit cell, compared to that of the semiconductor.
The details of processing such ceramics are also important. The compounds are extremely sensitive to small differences in thermal treatment and it is difficult to obtain two samples of the same (presumably) composition having identical electrical properties. Each individual particle of the powder so-produced has microscopic grains within each crystal composing the powder. Each grain is essentially single-crystal but is joined in random orientation to each of its neighbors. This alone reduces the current-carrying capacity by a factor of perhaps 100. In addition, each grain boundary is a poor conductor. But low current capacity is not the only problem. The ceramic is brittle, due in part to the randomly oriented grains, and it will deteriorate when exposed to water vapor. In addition, purity of the raw materials used is also a problem, since inclusion of even parts per billion of an impurity would cause formation of a non-superconducting composition on a microscopic scale within a given grain. Another problem with bulk (powder) materials is that the crystalline structure is layered (see above). It is suspected that current prefers to flow within the layers and that superconductivity breaks down in the direction perpendicular to those planes. If the layers could be coaxed into favored orientation, such a wire or strand could, in theory, carry much higher current densities.
Superconducting compositions are usually prepared by calcining carefully formulated mixtures of oxides. For example, to prepare the YBa.sub.2 Cu.sub.3 O.sub.6.3 superconducting phase, one weighs out 0.5 mol of Y.sub.2 O.sub.3, 2.0 mol of BaO, 3.0 mol of CuO, and mixes them thoroughly. The mixture is then calcined at elevated temperature in an oxygen-containing atmosphere whereupon the oxides undergo solid state reaction to form a single phase with superconducting properties at 78 K. Alternatively, one can choose compounds which decompose to form oxides which react to form the desired phase, when heated to elevated temperature.
Once the powder has been prepared, it can be handled by conventional means and processed to desirable forms. One such method employs a slurry of powder and methanol. By casting a uniform film on a suitable substrate such as sapphire, one can dry it, calcine it, and obtain a dense, uniform layer possessing superconducting properties. A micro- circuit can be etched in the film by laser ablation to obtain desired designs. However, this step mandates a reannealing step in oxygen atmosphere to restore the critical oxide stoichiometry required for superconductivity.
Another method to prepare a superconducting film, particularly for use, on a silicon substrate as an integrated circuit, has been to deposit thin layers of the appropriate metal oxides in specific order by electron-beam evaporation. Copper is first deposited, then barium, and then yttrium, all as oxides. The sequence is repeated 6-times to obtain an "18-layer" stack of the three ingredients having a total thickness of 0.6-0.7 microns. To complete the process, the specimens are then annealed in oxygen atmosphere for five minutes and then cooled at a rate of about 120.degree. C. per hour. It was necessary to deposit a buffer layer of inert zirconia on the silicon substrate, before the oxides were deposited, in order to prevent the oxides from reacting with the silicon substrate before the superconducting composition formed. The annealing step was shown to be extremely critical since the oxygen content in the film must be precisely maintained within certain (unknown) limits for the superconductivity state to prevail.
In a method using electron-beam evaporation, the new thallium-based compositions were deposited in films in sequential order under a partial oxygen pressure. The film was then subjected to two partial annealing steps because the thallium content must be carefully controlled. Such films were able to carry a current of about 110,000 A./cm.sup.2. They were deposited on several substrata, including sapphire, strontium titanate and silicon.
Another approach to preparation of superconducting films has been to employ compounds which are volatile and to cause them to decompose on a hot surface in a partial vacuum. This method, known as vapor phase epitaxy, is well suited for the preparation of integrated circuits on a silicon substrate and is capable of producing a superconductive monocrystalline film, using halogen compounds (or others) as the source materials, provided that suitable annealing in an oxygen atmosphere is carried out.
Still another method for preparing superconductors in useful form has been the formation of the ceramic composition by heating together specific mixtures of oxides. Once the superconducting composition had been formed, it was compacted into a bar. Said bar was then heated on a pedestal by a LASER until it melted, a seed crystal was added, and a fiber was drawn at a controlled rate. The prototype wire was able to carry 30,000 A/cm.sup.2 at 4 xK. before it failed. The composition used, Bi.sub.2 Sr.sub.3-x Cax Cu.sub.2 O.sub.8+y, was sintered, then reground and sintered again at least two more times, to achieve a uniform composition. The fiber so-produced was a single crystal but was subject to the shortcomings of all ceramic fibers, namely flexibility and ductility.
Another method to form a superconducting film has been to prepare a superconducting powder of Y.sub.1.0 Ba.sub.1.8 Cu.sub.3.0 O.sub.7-x composition, using conventional means. The initial preparation was checked for superconducting properties by measuring a pressed and sintered pellet. Once the material was found to have the desired properties, a powder slurry was made and the slurry was applied with a spin coater. The layer was dried and then fired in an oxygen atmosphere. Best films were obtained when fired at 940.degree.-1000.degree. C. If sapphire was used as the substrate, the adherence was such that the films could be ground and polished. One could then etch the film with a laser to obtain a desired geometry of superconducting lines, similar to those of a printed circuit.
All of the above methods and compositions given above for producing superconducting materials and forms are limited in the form of the superconductor that they are able to produce. For example, the electron-beam evaporation method or the vapor phase epitaxial growth method can only produce thin films which are superconductive at 78 K. Even the method which employs laser-melting of a ceramic bar to form a single crystal fiber has its limitations of size and form. The slurry method can produce a facsimile of an integrated circuit, but only if great effort is expended. None of the above methods can be adapted to the storage of energy for long periods of time.